The invention to be described hereinafter relates to processing of materials to obtain a purified product. In particular, there is described a novel method for the recovery of nearly pure calcium sulphate from a naturally occurring material such as the mineral gypsum, or from calcium sulphate-containing materials arising from an industrial process or technological exploitation of materials. Thus the invention provides for production of purified material from raw mineral sources or as part of a recycling process.
The chemical compound calcium sulphate, referred to hence as CaSO4, occurs in nature as the mineral gypsum. CaSO4 is a commodity product with a market-size that runs into several million tonnes per annum, worldwide. CaSO4 finds a wide range of applications in the construction, ceramics and pharmaceutical industries.
Most of the world""s supply of CaSO4 is obtained by open-cast mining of gypsum deposits. Gypsum is mined in nearly every country in the world, especially in the UK, France, Spain and the USA. The mining process is conventional: the ore is extracted by blasting (controlled explosion), followed by physical separation of high-grade gypsum ( greater than 98% by weight CaSO4) from lower-grade material. Typically, half of the material (by weight) recovered from a blast will be high-grade gypsum. The remaining lower-grade material, which still might contain up to 90% by weight CaSO4, is not further processed and common practice is to back-fill it on-site.
CaSO4 is also a by-product of the flue-gas desulphurisation process operated in modern fossil-fuel driven power-stations. In this process sulphur-oxyacids present in flue gases are removed by chemical reaction with a calcium oxide filter. The product is a low-grade CaSO4 that is contaminated with other materials derived from flue gases, including elemental sulphur.
Economic and environmental concerns are driving research into methods of converting low-grade CaSO4 into high-grade, or pure, CaSO4 and for its extraction from products of anthropomorphic origin as part of the recycling process.
Hitherto, conventional approaches have not revealed an economic solution to realise the desired conversion.
The standard chemists"" approach to purification is to identify a solvent which preferentially dissolves the target compound, thus permitting the insoluble contaminants to be removed using an appropriate physical process such as filtration or centrifugation. The first step in identifying a suitable solvent is to understand the chemical nature of the target compound.
CaSO4 is an ionic compound, or salt, which is nevertheless insoluble in water because the energy released by the hydration of the component calcium (Ca2+) and sulphate (SO42xe2x88x92) ions, that is by the creation of water-ion bonds, is less than the energy required to neutralise the ion-ion bonds within the solid and hence allow solidxe2x86x92solution phase transfer. Thus the natural order is to confer stability on the solid salt in preference to a solution of respective ions and one readily observes that the solid substance retains its integrity in contact with water. Therefore, this discouraging observation has left the purification of materials containing CaSO4 as something of a challenge to those of ordinary skill in the art.
This unique stability of CaSO4 in water contrasts with that for soluble salts such as sodium chloride (NaCl), where the energy released in the formation of water-ion bonds exceeds that of the ion-ion bonds in the crystal lattice, hence the crystal lattice breaks down and the ions dissolve. Nevertheless, calcium and sulphate ions are intrinsically soluble in water, as can be seen by comparison with other salts. Thus, for example, sodium sulphate (Na2SO4) and calcium chloride (CaCl2) are both highly water-soluble salts.
CaSO4 will, however, dissolve in aqueous solutions containing a calcium chelating agent. Chelating agents are a class of compounds which form very strong bonds with divalent metal ions (M2+), such Ca2+, much stronger than those formed between the metal-ion and water. The chemical properties of chelating agents are well-described in the scientific literature. A number of different chelating agents, with differing specifities for the particular metal ion they chelate, are available commercially. They find numerous applications in chemistry and biology as means of keeping otherwise insoluble salts of divalent metal-ions in solution.
The compound ethane 1,2-diamine N,N,Nxe2x80x2Nxe2x80x2 tetra-acetic acid (EDTA) is one example of a calcium chelating agent which is commercially available. In aqueous solution, at neutral or alkaline pH, EDTA reacts with the calcium ion to form the highly water-soluble calcium-EDTA complex, CaEDTAxe2x88x92. For example, at 20xc2x0 C. and pH 10 the reaction can be written as:
Ca2++EDTA3xe2x88x92CaEDTAxe2x88x92xe2x80x83xe2x80x83(Equation 1)
The equilibrium constant for the reaction is defined as,   K  =            [              CaEDTA        -            ]                      [                  Ca                      2            +                          ]            ⁡              [                  EDTA                      3            -                          ]            
At 20xc2x0 C. and pH 10, for example, K=1010dm3molxe2x88x921; that is the binding of Ca2+ to EDTA is essentially irreversible.
When calcium sulphate is mixed with an aqueous solution of EDTA at neutral or alkaline pH, therefore, the CaEDTAxe2x88x92 complex-ion dissolves and the sulphate ion follows it into solution as an independent entity. This is a spontaneous process and the resulting solution is thermodynamically stable and optically transparent. However, EDTA is unable to chelate calcium ions at low pH (i.e. below pH4). Therefore when this solution is acidified the CaEDTAxe2x88x92 complex-ion breaks down, Ca2+ and SO42xe2x88x92, recombine and CaSO4 precipitates.
The invention to be more particularly described hereinafter, provides the basis for a chemical technology which allows CaSO4 to be separated from other materials. This invention provides a chemical process for the purification of CaSO4 that utilises the ability of an aqueous solution of a chelating agent to selectively dissolve CaSO4. The ideal chelating agent is one which is soluble in water over a wide range of pH, but which chelates calcium only over a relatively narrow pH range. Thus according to the invention the initial extraction can be carried out at certain pH and CaSO4 can be recovered by titration to a different pH (usually a lower pH) following a mechanical treatment, such as centrifugation or filtration, to separate the aqueous chelate solution from insoluble material.
The aqueous solution of the chelating agent can be recycled for another round of CaSO4 extraction following titration back to the original pH.
Thus according to the present invention there is provided a method for the recovery of calcium sulphate from a crude or contaminated calcium sulphate source, in essentially pure form, by the method of dissolving the material in an aqueous solution of a calcium-chelating agent at a pH at which chelation occurs (the extraction step), followed by removal of insoluble residuum by a suitable physical technique such as, but not limited to, centrifugation or filtration (the separation step), followed by recovery of the calcium sulphate by acidification to a pH at which calcium sulphate, but not the chelating agent, precipitates (the recovery step).
The invention may be applied whatever the origin of the crude or contaminated material. Thus the invention is suitable for use on naturally occurring materials or those arising from the intervention of technology, including but not limited to, mineral deposits such as gypsum, and gypsum containing formations such as limestone, dolomitic limestones, shales and cap rock (an anhydrite-gypsum deposit found in Texas and Louisiana), to by-products of industrial processes such as the flue-gas desulphurisation process, contaminated wastes such as phospho-gypsum (a uranium contaminated material), construction industry debris, waste cladding materials and demolition rubble, and to calcium sulphate-containing commodity materials as part of the recycling process.
A wide variety of calcium chelating agents can be applied with varying levels of efficiency including, but not limited to ethane 1,2-diamine N,N,Nxe2x80x2Nxe2x80x2 tetra-acetic acid (EDTA) or 2-{(carboxymethyl) [2-trimethylamino)ethyl]amino}acetic acid, and sodium salts of such agents as well as the free acids are considered to equally effective in view of the ready mobility of the sodium ion in dissociating in aqueous solution.
A suitable chelating agent is a soluble polydentate, e.g. at least tridentate ligand, which may be an organic chelating compound modified by addition of or substitution with a solubilising group, e.g. a quaternary ammonium group, which is soluble in acid pH ranges, especially remaining soluble below pH 4.
Chelating groups include, but are not limited to, sulphonic and carboxylic groups, and the latter are preferred for purification of calcium sulphate.
Preferred chelating agents include the following: 4-(carboxymethyl)-2-(trimethylamino)pentane-1,5-dicarboxylic acid, 2-(carboxymethyl)-2-(trimethylamino)butane-1,4,dicarboxylic acid, 2-(carboxymethyl)-3-(trimethylamino)butane-1,4-dicarboxylic acid, and the like.
In the method of the invention, preferably the solution is made acidic, for the purpose of recovery of the calcium sulphate, by the addition of any acidic substance including, but not limited to, sulphuric acid.
Preferably the acidic solution of the chelating agent obtained following recovery of the calcium sulphate is made neutral or alkaline once again by the addition of any basic substance including, but not limited to, sodium hydroxide or preferably calcium hydroxide.
Preferably the aqueous solution of the chelating agent is recycled from a previous extraction, following titration to neutral or alkaline pH by the method.
The extraction, separation and recovery stages may be carried out at any temperature considered appropriate by those skilled in the art.
The invention further provides an apparatus for the purification of calcium sulphate by these methods of the invention, and comprising a vessel for receiving crude or contaminated material containing calcium sulphate to be purified, at least one calcium chelating reagent and an aqueous medium, whereby said material and said reagent are intimately admixed in said aqueous medium to form an extraction liquor, means for separating insoluble materials from said extraction liquor, means for adjusting the pH of the extraction liquor by supply of acidic or basic media into said vessel, and means for recovery of purified calcium sulphate.
The apparatus may comprise a plurality of reaction vessels each dedicated to a particular stage of the process, e.g. an extraction vessel, a separation vessel, a recovery vessel, and each may be associated with and operatively connected to separate reagent supply vessels. The extraction vessel may be equipped with agitation means or may be adapted to achieve mixing in other ways e.g. static mixers are known in the chemical industry whereby reagent supply lines and internal baffles and conduits are provided according to a design predetermined to promote intermingling and mixing of reagent fluids.
The separation vessel may be adapted to operate under centrifugal separation principles as a matter of preference but also may be adapted to decant extraction liquor from insoluble materials retained for example by means of a weir or the like. Alternatively filter means may be provided within the separation vessel to achieve the same purpose.
The recovery vessel may also include filter means or be adapted to decant liquor after the desired precipitation of calcium sulphate material has occurred. The recovery vessel may also be adapted to provided for at least one washing step for the precipitate of calcium sulphate.
The apparatus may also be equipped with recycle lines for returning recovered reagents to the process.